[llvm-mc] - Properly set the the address align field of the compressed sections.

[grimar]

Summary

About the compressed sections spec says:
(https://docs.oracle.com/cd/E37838_01/html/E36783/section_compression.html)
sh_addralign fields of the section header for a compressed section
reflect the requirements of the compressed section.

Currently, llvm-mc always puts uncompressed section alignment to sh_addralign.
It is not correct. zlib styled section contains an Elfxx_Chdr header,
so we should either use 4 or 8 values depending on the target
(Uncompressed section alignment is stored in ch_addralign field of the compression header).

GNU assembler version 2.31.1 also has this issue,
but in 2.32.51 it was already fixed. This is how it was found
during debugging of the https://bugs.llvm.org/show_bug.cgi?id=40482
actually.

[dblaikie]

Soundsn good to me - thanks!

lib/MC/ELFObjectWriter.cpp

void ELFWriter::writeSectionData(const MCAssembler &Asm, MCSection &Sec,
 
   bool ZlibStyle = MAI->compressDebugSections() == DebugCompressionType::Z;
   if (!maybeWriteCompression(UncompressedData.size(), CompressedContents,
                              ZlibStyle, Sec.getAlignment())) {
     W.OS << UncompressedData;
     return;
   }
 
-  if (ZlibStyle)
+  if (ZlibStyle) {
     // Set the compressed flag. That is zlib style.
     Section.setFlags(Section.getFlags() | ELF::SHF_COMPRESSED);
-  else
+    // Alignment field should reflect the requirements of
+    // the compressed section header.
+    Section.setAlignment(is64Bit() ? 8 : 4);
+  } else {
     // Add "z" prefix to section name. This is zlib-gnu style.
     MC.renameELFSection(&Section, (".z" + SectionName.drop_front(1)).str());
+  }
   W.OS << CompressedContents;
 }
 
 void ELFWriter::WriteSecHdrEntry(uint32_t Name, uint32_t Type, uint64_t Flags,
                                  uint64_t Address, uint64_t Offset,
                                  uint64_t Size, uint32_t Link, uint32_t Info,
                                  uint64_t Alignment, uint64_t EntrySize) {
   W.write<uint32_t>(Name);        // sh_name: index into string table

test/MC/ELF/compression.s

 // REQUIRES: zlib
 // Check zlib-gnu style
 // RUN: llvm-mc -filetype=obj -compress-debug-sections=zlib-gnu -triple x86_64-pc-linux-gnu < %s -o %t
 // RUN: llvm-objdump -s %t | FileCheck --check-prefix=CHECK-GNU-STYLE %s
 // RUN: llvm-dwarfdump -debug-str %t | FileCheck --check-prefix=STR %s
 // RUN: llvm-mc -filetype=obj -compress-debug-sections=zlib-gnu -triple i386-pc-linux-gnu < %s \
 // RUN:     | llvm-readobj -symbols - | FileCheck --check-prefix=386-SYMBOLS-GNU %s
 
 // Check zlib style
 // RUN: llvm-mc -filetype=obj -compress-debug-sections=zlib -triple x86_64-pc-linux-gnu < %s -o %t
 // RUN: llvm-objdump -s %t | FileCheck --check-prefix=CHECK-ZLIB-STYLE %s
 // RUN: llvm-dwarfdump -debug-str %t | FileCheck --check-prefix=STR %s
-// RUN: llvm-mc -filetype=obj -compress-debug-sections=zlib -triple i386-pc-linux-gnu < %s \
-// RUN:     | llvm-readobj -symbols - | FileCheck --check-prefix=386-SYMBOLS-ZLIB %s
-// RUN: llvm-readobj -sections %t | FileCheck --check-prefix=ZLIB-STYLE-FLAGS %s
+// RUN: llvm-readobj -sections %t | FileCheck --check-prefixes=ZLIB-STYLE-FLAGS,ZLIB-STYLE-FLAGS64 %s
+
+// RUN: llvm-mc -filetype=obj -compress-debug-sections=zlib -triple i386-pc-linux-gnu < %s -o %t
+// RUN: llvm-readobj -symbols -sections %t \
+// RUN:   | FileCheck --check-prefixes=386-SYMBOLS-ZLIB,ZLIB-STYLE-FLAGS,ZLIB-STYLE-FLAGS32 %s
 
 // Don't compress small sections, such as this simple debug_abbrev example
 // CHECK-GNU-STYLE: Contents of section .debug_abbrev:
 // CHECK-GNU-STYLE-NOT: ZLIB
 // CHECK-GNU-STYLE-NOT: Contents of
 
 // CHECK-GNU-STYLE: Contents of section .debug_info:
 

 // Check that debug_line section was not renamed, so it is
 // zlib-style, not zlib-gnu one. Check that SHF_COMPRESSED was set.
 // ZLIB-STYLE-FLAGS:      Section {
 // ZLIB-STYLE-FLAGS:        Index:
 // ZLIB-STYLE-FLAGS:        Name: .debug_str
 // ZLIB-STYLE-FLAGS-NEXT:   Type: SHT_PROGBITS
 // ZLIB-STYLE-FLAGS-NEXT:   Flags [
 // ZLIB-STYLE-FLAGS-NEXT:     SHF_COMPRESSED
+// ZLIB-STYLE-FLAGS-NEXT:     SHF_MERGE
+// ZLIB-STYLE-FLAGS-NEXT:     SHF_STRINGS
+// ZLIB-STYLE-FLAGS-NEXT:   ]
+// ZLIB-STYLE-FLAGS-NEXT:   Address:
+// ZLIB-STYLE-FLAGS-NEXT:   Offset:
+// ZLIB-STYLE-FLAGS-NEXT:   Size:
+// ZLIB-STYLE-FLAGS-NEXT:   Link:
+// ZLIB-STYLE-FLAGS-NEXT:   Info:
+// ZLIB-STYLE-FLAGS32-NEXT: AddressAlignment: 4
+// ZLIB-STYLE-FLAGS64-NEXT: AddressAlignment: 8
 
 // 386-SYMBOLS-ZLIB: Name: .Linfo_string0
 // 386-SYMBOLS-ZLIB-NOT: }
 // 386-SYMBOLS-ZLIB: Section: .debug_str
 
   .section  .debug_line,"",@progbits
 
   .section  .debug_abbrev,"",@progbits